I. Field of the Invention
The present invention relates generally to iontophoresis devices for the transdermal delivery of active agents by the use of an applied electromotive force (emf). More particularly, the present invention is directed to an electrode assembly for an associated iontophoresis device which incorporates an accurate, positive circuit breaking element that stops electrical activity in the iontophoresis device after the administration of a given total quantity of active agent.
II. Related Art
It is known to construct an iontophoresis device designed to administer a given is total quantity of active agent based on the consumption of a consumable electrode leading to a break in electrical conductivity in the iontophoresis circuit. As used throughout this specification, the terms “consumable”, “consumed”, or the like, refer to materials that are oxidized or reduced in the operation of the corresponding iontophoresis device. Such arrangements are illustrated and described, for example, in U.S. Pat. No. 5,320,731 in which an iontophoresis device is constructed having a signal generator connected to a pair of electrodes, one of which is a limiting consumable electrode, i.e., one containing a limited quantity of material preferentially electrochemically consumed (oxidized or reduced) in relation to the other materials of the iontophoresis device. The quantity of electricity necessary for complete reaction of the material designed to be electrochemically consumed is also designed to correspond to the quantity necessary to deliver the desired amount of active material to be administered by the iontophoresis device.
The consumable electrode material is applied in the form of a coating on an insulating surface or, alternatively, on a conducting support which is unreactive, i.e., does not oxidize or reduce in the environment of the device. When the consumable material has been reacted, the material becomes non-conducting and so the current path between the pair of electrodes is severed and delivery by iontophoresis stops.
While devices heretofore developed using the principle of incorporating a consumable electrode to limit agent delivery by iontophoresis have been based on sound theory, most have had certain drawbacks which have limited their useful application. Examples of such prior art consumable electrode configurations are represented in rudimentary schematic form in FIGS. 1(a)-1(c) which are side elevational or sectional views depicting the layered structure of prior art consumable electrode models of the circuit breaking type. These models are described as consumable (oxidizable) anode assemblies or partial electrophoresis systems for the delivery of a therapeutic agent but, of course, the concepts illustrated apply equally to consumption by reduction in cathode systems.
In FIG. 1(a), an anode assembly is shown generally at 10 and includes partial top layer 12 which represents the active ingredient-containing pad, a sacrificial metal-containing electrode layer 14 and a base layer 16 which selectively may be a non-electrically conducting (insulating) material or an electrically conductive material that is not consumed in the system. The theoretical concept is that when the consumable anode material located beneath the active ingredient pad 12 is fully consumed, a break will occur in the electrical circuit of the device. This is illustrated in FIG. 1(b) where the portion 18 of the consumable metal-containing layer 14 is indicated as having been consumed thereby breaking circuit continuity at 20. This, of course, represents the ideal situation in which the portion 18 is entirely consumed prior to the breaking or failing of the circuit.
It is well known, however, that layers of material, and particularly thin layers of material, under such circumstances are generally consumed at random which allows consumption in a manner which may well isolate a portion of the layer from the rest thereby precluding total consumption of the consumable material, and thereby also causing premature failure of the electrode. This situation is illustrated in FIG. 1(c) wherein a central portion of the layer 14 is shown consumed at 22 and, although a plan view is not shown, this consumed central portion is deemed to extend all the way across the layer thereby isolating distal portion 24 of the sacrificial material prior to full consumption as at 18 in FIG. 1(b).
The situation illustrated in FIG. 1(c) can be avoided by making the base layer 16 electrically conductive but inert with respect to being oxidized or reduced. This will allow all the desired sacrificial material to be consumed as at 18 in FIG. 1(b); however, even after this takes place, the circuit remains intact as conduction is maintained along the electrically conductive base layer. This has a potentially serious drawback in that any water in the system may thereafter be oxidized or reduced producing corresponding pH changes in the system at the surface of the layer 16. Such pH changes in the system are quite undesirable because they can cause adverse reactions with the skin of a patient to which the iontophoresis device has been applied and prevention of just such changes in pH has been a long-sought goal in the operation of such devices. Additionally, if the drug is dissolved in a water solution (as is typical) there is an abundance of water present and an additional amount, possibly an overdosage of drug will be delivered in accordance with the amount of water electrochemically consumed by oxidation or reduction.
Of course, if the base material 16 is not only conductive, but is a material that will be oxidized or reduced in the device, then this material too will be consumed in an unpredictable fashion again infusing an uncertainty as to the amount of active material that will actually be delivered by the device.
Accordingly, there is a need to provide more accurate control of the circuit breaking characteristic associated with sacrificial or consumable electrode materials in iontophoresis devices.